I'm a dancer and after and injury, I lost my back flexibility. To get it back I stretched every day (butterfly, straddle leans, pigeons while leaning forwards and backwards) in addition to doing bridges and walking with them and doing rocking horses (lay on your stomach and curl your legs up so you can grab them. Pull up taking your chest off the floor and hold for 30 seconds, release and rest for 30 seconds. Then repeat while rocking forwards and backwards) After about a week I regained my flexibility. Good luck! :)